99 lines
4.3 KiB
TeX
99 lines
4.3 KiB
TeX
\subsection{Surface scattering models}
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\label{sec:bsdfs}
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Surface scattering models describe the manner in which light interacts
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with surfaces in the scene. They conveniently summarize the mesoscopic
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scattering processes that take place within the material and
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cause it to look the way it does.
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This represents one central component of the material system in Mitsuba---another
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part of the renderer concerns itself with what happens
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\emph{in between} surface interactions. For more information on this aspect,
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please refer to Sections~\ref{sec:media} and \ref{sec:subsurface}.
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This section presents an overview of all surface scattering models that are
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supported, along with their parameters.
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\subsubsection*{Correctness considerations}
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\begin{figure}[b!]
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\centering
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\vspace{-5mm}
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\includegraphics[width=15cm]{images/glass_explanation.pdf}
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\vspace{-5mm}
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\caption{
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\label{fig:glass-explanation}
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Some of the scattering models in Mitsuba need to know
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the indices of refraction on the exterior and interior-facing
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side of a surface.
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It is therefore important to decompose the mesh into meaningful
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separate surfaces corresponding to each index of refraction change.
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The example here shows such a decomposition for a water-filled Glass.
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}
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\end{figure}
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A vital consideration when modeling a scene in a physically-based rendering
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system is that the used materials do not violate physical properties, and
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that their arrangement is meaningful. For instance, imagine having designed
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an architectural interior scene that looks good except for a white desk that
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seems a bit too dark. A closer inspection reveals that it uses a Lambertian
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material with a diffuse reflectance of $0.9$.
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In many rendering systems, it would be feasible to increase the
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reflectance value above $1.0$ in such a situation. But in Mitsuba, even a
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small surface that reflects a little more light than it receives will
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likely break the available rendering algorithms, or cause them to produce otherwise
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unpredictable results. In fact, we should rather change the lighting setup and
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then \emph{reduce} the material's reflectance, since it is quite unlikely that
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we could find a real-world desk with a reflectance as high as $0.9$.
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As an example of the necessity for a meaningful material arrangement, consider
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the glass model illustrated in \figref{glass-explanation}. Here, careful thinking
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is needed to decompose the object into boundaries that mark index of
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refraction-changes. If this is done incorrectly and a beam of light can
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potentially pass through a sequence of incompatible index of refraction changes (e.g. $1.00\to 1.33$
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followed by $1.50\to1.33$), the output is undefined and will quite likely
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even contain inaccuracies in parts of the scene that are some distance
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away from the glass.
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\subsubsection*{BSDFs}
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To achieve realistic results, Mitsuba comes with a library of both
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general-purpose surface scattering models (smooth or rough glass, metal,
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plastic, etc.) and specializations to particular materials (woven cloth,
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masks, etc.). Some model plugins fit neither category and could be described
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as \emph{modifiers} that are applied on top of one or more scattering models.
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Throughout the documentation and within the scene description
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language, the word \emph{BSDF} is used synonymously with the term ``surface
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scattering model''. This is an abbreviation for \emph{Bidirectional
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Scattering Distribution Function}, a more precise technical
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term.
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In Mitsuba, BSDFs are
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assigned to \emph{shapes}, which describe the visible surfaces in
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the scene. In the scene description language, this assignment can
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either be performed by nesting BSDFs within shapes, or they can
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be named and then later referenced by their name.
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The following fragment shows an example of both kinds of usages:
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\begin{xml}
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<scene>
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<!-- Creating a named BSDF for later use -->
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<bsdf type=".. BSDF type .." id="myNamedMaterial">
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<!-- BSDF parameters go here -->
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</bsdf>
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<shape type="sphere">
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<!-- Example of referencing a named material -->
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<ref id="myNamedMaterial"/>
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</shape>
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<shape type="sphere">
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<!-- Example of instantiating an unnamed material -->
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<bsdf type=".. BSDF type ..">
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<!-- BSDF parameters go here -->
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</bsdf>
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</shape>
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</scene>
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\end{xml}
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It is generally more economical to use named BSDFs when they
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are used in several places, since this reduces Mitsuba's internal
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memory usage.
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